1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit formed of thin film transistors (hereinafter referred to as TFTs) using a method of effectively removing a metallic element which promotes crystallization of an amorphous silicon film. For example, the present invention relates to an electro-optical device represented by an active matrix liquid crystal display device having a pixel portion and a driver circuit provided on one substrate, and to an electronic apparatus having such an electro-optical device mounted thereon as a component.
Also, the present invention relates to a light emitting device using a light emitting element which utilizes a light emitting material in which EL (electro luminescence) is obtained as the light emitting material (hereafter referred to as an EL light emitting device, an EL element, and an EL material, respectively). Note that EL materials which can be used in the present invention include all light emitting materials in which light is emitted via singlet state excitation, triplet state excitation, or both types of excitation (phosphorescence and/or fluorescence).
It is to be noted that, as used herein, the term “semiconductor device” means any device which can function by utilizing semiconductor characteristics, and includes not only any single element as a TFT but also any electro-optical device formed using TFTs, any electronic apparatus having such an electro-optical device mounted thereon as a component, and any semiconductor circuit.
2. Description of the Related Art
A TFT using a semiconductor thin film is utilized in various kinds of integrated circuits. Semiconductor thin films include amorphous silicon films and crystalline silicon films. An amorphous silicon film is easily formed, and thus, has excellent productivity. However, since the electric characteristics of its TFT is low, the operation speed is slow, and therefore, it can not be used in an active matrix liquid crystal display device having an integral, peripheral driver circuit, and can not form various kinds of integral circuits. Therefore, in such a case, a crystalline silicon film having better characteristics is used.
As methods of forming a crystalline silicon film, there are thermal annealing and laser annealing. Thermal annealing requires a high-temperature process of 600° C. or above, and thus, it can not be applied to a glass substrate which is low-cost and permits large-area device. Thermal annealing has another problem that the process time is long. On the other hand, with regard to laser annealing, though it can realize a process which does not cause thermal damage to a substrate, it has problems such as insufficient evenness of crystallization, insufficient repeatability, and insufficient crystallinity. One way to solve these problems is to promote crystallization using a predetermined metallic element.
For example, Japanese Patent Application Laid-open No. Hei 7-130652 applied by the assignee of the present invention discloses a method which uses thermal annealing but suppresses the crystallization temperature to 600° C. or lower which is applicable to a glass substrate. In this method, thermal annealing is performed with a metallic element represented by Ni introduced in an amorphous silicon film, and a crystalline silicon film having sufficient crystallinity can be obtained.
In a case a method is used where a predetermined metallic element is used to promote crystallization, since crystallization proceeds as the metallic element diffuses and moves, the metallic element for promoting crystallization remains in the crystalline silicon film. As a result, the metallic element deposits around the vicinity of the surface of the crystalline silicon film to cause leak at a junction. Further, the metallic element forms a deep level to become a center of recombination or generation of carriers. Therefore, there is a problem that the stability and the reliability of the electric characteristics of the TFT are deteriorated. In order to solve the problem, various kinds of gettering technologies have been developed to remove or decrease the metallic element.
Gettering is performed by, for example, after the amorphous silicon film is crystallized into a crystalline silicon film using the metallic element and a portion to be a device region is covered with a mask layer such as an oxide film, heavily doping an element of the group 15 such as P, which is effective in gettering, in the remaining region other than the device region to make that region other than the device region promote gettering (hereinafter referred to as a gettering site), or, after a portion to be a device region is masked in a similar way, forming thereon a silicon film containing a high concentration of an element of the group 15 such as P to be a gettering site. However, since these methods require processes of forming and patterning a film to be a mask layer, the number of the masks increases, the manufacturing cost increases, and the productivity is lowered.
Another way to perform gettering is, for example, to make a source region and a drain region of a device a gettering site. In this method, though the number of the masks can be decreased since patterning for the gettering is not necessary, the capacity of the gettering site is limited and the gettering efficiency is lowered to some extent. Further, since the element of the group 15 such as P to be a donor is doped also in a p-channel TFT, an excess amount of ions to be an acceptor has to be doped, which causes increase in the manufacturing cost and lowering of the productivity.